Blender 2.8 Less Saturated Colors in Material
If you want the render to show exactly the color of it, you will need to directly connect an RGB node to the surface socket of output node to avoid any other calculation of final color.

And if my memory is correct, default view transform calculation behavior is changed in 2.80 Cycles. Did you accidentally change it away from default setting?

If some of these value is not as same as the screen shot, then it will cause the color picker not able to retrieve the original color.